The present invention relates generally to semiconductor fingerprint detectors, and more particularly to improvements in capacitive elements used to sense the fingerprint pattern of a finger pressed against a sensing surface.
A capacitive distance sensor is disclosed in commonly-assigned U.S. Pat. No. 6,114,862 by Tartagni et al., the disclosure of which is hereby incorporated by reference. The Tartagni et al. patent discloses a fringing capacitive sensing technique in which two side-by-side capacitor plates are disposed just beneath a sensing surface at each of many cell locations in a two-dimensional array of sensor cells or xe2x80x9cpixelxe2x80x9d cells. The side-by-side capacitor plates of each pixel cell form part of a feedback circuit having an effective feedback capacitance that is modulated by the presence of live skin close to the sensing surface.
The technique disclosed in the Tartagni et al. patent is an active sensing technique in which transistor circuitry is employed at each pixel location to assist in the detection and output signal generation of the signal from each pixel cell. Passive capacitive sensor cells are also known in the art, as disclosed by example in U.S. Pat. No. 4,353,056.
The present invention provides an improved capacitive pixel for a fingerprint sensing device. Active sensing is used in a solid-state pixel structure that directly senses the electric field potential of a fingerprint of a user""s finger pressed against a sensing surface. A floating capacitor plate is disposed beneath the sensing surface to define a sensing capacitor with the skin of the user""s finger. The capacitance of the sensing capacitor varies with the proximity of the skin to the sensing surface so that fingerprint ridges produce a higher capacitance than fingerprint valleys. First and second coupling capacitor plates are disposed beneath the floating capacitor plate to define first and second coupling capacitors. The coupling capacitors are connected in series between a reference voltage source and an input to a charge integrator. When a reference voltage signal is applied to the pixel, the charge integrator generates an output signal that is a function of the capacitance of the sensing capacitor.
Accordingly, a compact pixel structure can be provided in an array of pixels to enable capturing of a high resolution fingerprint image. The structure gives a high signal-to-noise level response and is relatively insensitive to parasitic capacitive effects.